Over the years, the study of machine dynamics has lead to the development of vibration-damping systems for reducing undesirable vibration modes produced when operating a machine. In order for machines to transform (or transfer) energy, they typically have a number of fixed and moving bodies interposed between a source of power and an area where work is to be done. The bodies serve to adapt one to the other. Electric motors transform electrical energy into mechanical energy. Gasoline engines have connecting rods and crankshafts that act as a machine to transfer combustion energy into drive train energy. Stirling engines convert heat into reciprocating piston motion within a thermodynamic gas environment, the piston working on the thermodynamic gas to create mechanical power. Each of these devices when operating produces vibration. Stirling cryogenic coolers convert electrical energy into reciprocating piston motion that operates on a thermodynamic gas via a reciprocating displacer to produce a cool region.
With nearly all types of machines, vibration is caused by operation of the machine. For many machines, the vibration involves some form of reciprocating motion within the machine. It is frequently desirable to eliminate one or more components of the vibration that are created during operation of a machine. Many devices have been created for reducing, or eliminating, machine vibration. For example, counterweights are used on a crankshaft of an internal combustion engine. However, many of these balance systems are complex and cumbersome. Some even require their own drive motor (active system).
One exemplary area where simple and lightweight balance systems are in need of significant improvements is the field of linear motion machines. One exemplary linear motion machine is a free piston Stirling machine, such as a free piston Stirling cycle engine. A typical free piston Stirling engine contains a single displacer and a single power piston that cooperate in fluid communication via a thermodynamic working gas. Such an engine construction can be resolved into a machine vibration problem that principally has a two-dimensional vibration component. Such machines have the simplest of controls, but are inherently unbalanced. The reciprocating masses cooperate through the working gas, transmitting alternating forces while within a sealed vessel. Typically, operation of such a Stirling machine can produce large unbalanced dynamic vibration forces that require use of a large mounting structure to absorb forces produced during operation. Alternatively, sophisticated suspension arrangements are required to isolate the machine from its mounting structure. However, these systems frequently prove too complex and heavy where it is necessary that the system be portable and lightweight. For example, use of these devices for space exploration and remote site usage requires that the devices be constructed to have a minimized total weight.
Therefore, there is a need to provide an improved balance system for use with vibrating machines which provides a needed counterbalance mass with a device having a reduced overall mass. Furthermore, there is a need to provide such a counterbalance mass in a manner which can be easily tuned to accommodate specific operating frequencies of the linear motion machine.
The present invention arose from an effort to develop a passive balance system that is relatively low in cost, is relatively light in weight for a particularly sized counterbalance mass, has vibration characteristics that can be easily tuned to a particular machine operating speed by changing the counterbalance mass and/or the spring constant, and can be easily mounted onto an existing machine along a desired line of vibration to be counterbalanced in one of several manners.